1. Technical Field
The present invention relates to a waveguide amplifier and, more particularly, to a waveguide amplifier including adiabatic regions within the waveguide structure to improve the efficiency of the amplifier.
2. Description of the Prior Art
In the field of optical communications, it has become well-known to provide an optical amplifier by forming a rare-earth (i.e., erbium) doped waveguide within the surface of an optical substrate. An article entitled "Er-diffused Ti:LiNbO.sub.3 waveguide laser of 1563 and 1576 nm emission wavelengths" by P. Becker et al. appearing in Applied Physics Letters, Vol., 61, No. 11, September 1992, describes one such exemplary structure. A conventional prior art Er-doped waveguide amplifier is illustrated in FIG. 1. Amplifier 10 comprises a doped waveguide 12 (such as, for example, a co-doped erbium and TiO.sub.2) formed in the top surface 14 of an optical substrate 16 (such as, for example, lithium niobate). A first reflective surface 18 is formed along the entrance port of substrate 16 and a second reflective surface 20 is formed along the exit port of substrate 16. An optical source 22 provides a pump signal Pp, at a predetermined wavelength .lambda..sub.p that is coupled through first reflective surface 18 and into waveguide 12. Pump signal P.sub.p will thereafter propagate along the length L of waveguide 12 and be reflected back along waveguide 12 towards first reflective surface 18. Waveguide amplifier 10 may be used as either an amplifier, or a laser at an operating wavelength .lambda..sub.p, which belongs to the emission spectrum of the stimulated rare earth ions. When used as a laser, the doping concentration within waveguide 12 needs to be relatively high (for example, several thousand parts epr million) in order to achieve the necessary gain over the length L of waveguide 12. A problem results in that the high doping level leads to ion cluster formation within waveguide 12, which in turn reduces the conversion efficiency (.lambda..sub.p .fwdarw..lambda..sub.s) of the amplifier/waveguide, yielding both lower gain and output power for the signal.
One alternative to maintain the inversion efficiency is to reduce the doping level within the waveguide and increase the length, L, of the waveguide structure. In order to use an optical substrate of conventional size, the increase in length may be achieved by using a "zig-zag", spiral, serpentine, or other appropriate pattern, across the substrate surface. However, while the waveguide length is increased, such a pattern necessarily increases the complexity of the device fabrication. Moreover, such a design has not been used successfully in the formation of a laser since the cavity increase also increases the number of modes (due to the decrease in free spectral range (FSR)).
Therefore, a need remains in the prior art for a waveguide design with improved gain-power efficiency.